When video is streamed over the Internet and played back through a Web browser or media player, the video is delivered in digital form. Digital video is also used when video is delivered through many broadcast services, satellite services and cable television services. Real-time videoconferencing often uses digital video, and digital video is used during video capture with most smartphones, Web cameras and other video capture devices.
For standard dynamic range (“SDR”), digital video represents common colors in a relatively narrow range of brightness. Brightness can be measured in candelas per square meter (cd/m2), which indicates luminous intensity per unit area. This unit of luminous intensity per unit area is called a “nit.” A typical SDR display device may represent colors from pale colors through colors that are relatively vivid, in a brightness range from 0 nits to 100 nits. More recently, display devices having high dynamic range (“HDR”) have been introduced. A typical HDR display device may represent colors in a wider color gamut (potentially representing colors that are more vivid or saturated) and in a larger brightness range (e.g., up to 1500 nits or 4000 nits). Video produced for playback on an HDR display device can have an even larger brightness range (e.g., 0 nits to 10,000 nits).
In a conventional production chain for HDR video processing, after input HDR video is captured using a camera or set of cameras, one or more production tools perform “content mastering” operations on the input HDR video. The content mastering process allows an editor to adjust dynamic range, average brightness, contrast, color tone, and/or other attributes of the HDR video for artistic effect or to improve quality of the HDR video as perceived by a viewer. The dynamic range and color gamut of the input HDR video are typically reduced. After content mastering operations, the HDR video is encoded. The encoded HDR video is distributed in any of various ways (e.g., streaming over a network, broadcast, via disk media). A video playback system receives the encoded HDR video, decodes the HDR video, and renders sample values of the HDR video for display on a display device. During the rendering process, the video playback system typically makes additional adjustments to the sample values of the HDR video, e.g., to adapt to the peak brightness of the display device and/or user settings.
Typically, content mastering operations in such a production chain attempt to optimize HDR video for display on a reference display device in a reference viewing environment. The reference viewing environment is a hypothetical viewing environment (e.g., one that assumes the HDR video will be played back in a dark room). The reference display device is a hypothetical display device, which has capabilities based on assumptions about the display device that will actually be used to play back the HDR video. Often, the reference display device is assumed to have a peak brightness that is typical for an HDR display device (e.g., 1000 nits, 1500 nits). During content mastering for a reference display device, details in bright sample values of HDR video may be lost. For example, if the peak brightness of the reference display device is 1500 nits, bright sample values above 1500 nits may clipped (that is, set to 1500 nits), or bright sample values above a certain threshold value (such as 1400 nits) may be compressed to a very small range. Such loss of details for bright highlights of HDR video due to content mastering operations can adversely affect the quality of the viewing experience, even if the display device used to play back the HDR video matches the reference display device.